HR: 14:40h
AN: B13E-05    [Abstracts]
TI: Preliminary experimental results for trace element uptake in carbonates: Pb2+ in calcite and U6+ in aragonite at various growth rates and temperatures
AU: * Gabitov, R I
EM: gabitr@rpi.edu
AF: Rensselaer Polytechnic Institute, Rensselaer Polytechnic Institute Department of Earth & Environmental Sciences 110 8th St., JSC 1W19 , Troy, NY 12180 United States
AU: * Gabitov, R I
EM: gabitr@rpi.edu
AF: Institute of Experimental Mineralogy, Institutskii prospect, Chernogolovka, 142432 Russian Federation
AU: Watson, B E
EM: watsoe@rpi.edu
AF: Rensselaer Polytechnic Institute, Rensselaer Polytechnic Institute Department of Earth & Environmental Sciences 110 8th St., JSC 1W19 , Troy, NY 12180 United States
AB: The surface of a crystal in equilibrium with surrounding fluid can have a composition and structure that differs from the bulk crystal. If the growth rate is fast relative to the diffusive equilibrium time, then the crystal surface composition may be partially "captured" by the newly-formed lattice. The extent of this growth entrapment increases with crystal growth rate and is suppressed by diffusive equilibration between the surface and the bulk lattice of the crystal. Partitioning of Sr into calcite is probably affected by this entrapment mechanism. To explore the relevance of this idea to other elements and other carbonates we conducted experiments on Pb$^{2+}$ in calcite and U$^{6+}$ in aragonite, for which relatively few data exist. There are, however, some intriguing indications in the published data that show the concentration of Pb$^{2+}$ is higher at the calcite surface relative to the bulk lattice; U$^{6+}$ partitioning into aragonite decreases with increasing growth rate. Both these previous studies involved the use of bulk analytical techniques; we, in contrast, performed in situ measurements on single crystals of known growth rate. The carbonate crystals were grown by two different mechanisms: 1) precipitation of calcite on a plate (pre-coated with calcite), using a steady flow of CaCl$_{2}$ - PbCl$_{2}$ and Na$_{2}$CO$_{3}$ solutions mixed just before passage through a tube and dripped onto a plate (cave-type experiments, I=0.02); 2) growth of calcite and aragonite from a CaCl$_{2}$ - NH$_{4}$Cl (or NaCl) - PbCl$_{2}$ (or UO$_{2}$[NO$_{3}$]$_{2}$) solution by diffusion of CO$_{2}$ from an ammonium carbonate source (drift experiments, I=0.52-0.58). The growth rate of individual crystals was determined by comparison of final size with duration of the experiment. The precipitated crystals show significant variation in size even when the runs have the same input rate of CaCO$_{3}$ components. The cave-type runs at 25$\mp$2, 40$\mp$1, and 50$\mp$1$\deg$C yielded 15-40 $\mu$m calcites, but in the drift experiments the aragonite crystals attained sizes up to 500 $\mu$m. Electron microprobe analysis across the large crystals shows that the concentration of U is higher in the center and decreases toward the edge. This is probably due to the cube root dependence of linear growth on volume change of the growing crystals. We also observed that increasing aragonite growth rate (V, nm/s) enhances U partitioning between the bulk crystal and the liquid, K$_{d}$$^{bulk/liquid}$=(U/Ca)$_{bulk}$/(U/Ca)$_{liquid}$ (in contrast to the previous study). K$_{d}$$^{bulk/liquid}$ = 0.65 to 1.91$\mp$0.08 when log(V)=-0.73 to 0.31 at 53$\mp$3$\deg$C in the drift-type run (I=0.58). This evidence supports the idea that U is enriched at the calcite surface relative to the bulk crystal during crystal growth. The Pb partition coefficient varies from 0.01 to 1.26. At present time we are characterizing the parameter that responsible for this broad interval of K$_{d}$ variation. Uranium partitioning into aragonite decreases with increasing temperature. Our preliminary data can be described by log(K$_{d}$) versus 10$^{4}$/T(K) plot with the slope equal to 0.064, corresponding to $\Delta$H$^{0}$ of -12.29 (kJ/mol).
DE: 4825 Geochemistry
DE: 1040 Isotopic composition/chemistry
DE: 1045 Low-temperature geochemistry
DE: 1050 Marine geochemistry (4835, 4850)
DE: 1065 Trace elements (3670)
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting